1. Field of the Invention:
The present invention relates generally to a thread form for tubular connections, and, more specifically, to threaded connections of the type used for securing flow conduits to form a desired continuous flow path.
2. Description of the Prior Art:
A variety of threaded connections are known in the prior art for joining flow conduits in an end-to-end relationship to form a continuous flow path for transporting fluid. For example, such threaded connections are used in pipe strings employed for the production of hydrocarbons and other forms of energy from subsurface earth formations. Examples of such pipe strings include drill pipe, well casing and production tubing, known commonly as "oil field tubular goods." Other applications for the threaded connections of the invention include horizontal/trenchless drilling operations. These are non-oilfield applications associated with the construction industry. All of these type goods employ threaded connections of the type under consideration for connecting adjacent conduit sections or pipe joints.
There have been numerous advances in thread technology in recent years. Re. Pat. No. 30,647 issued to Blose in 1981 disclosed a tubular connection having a unique thread form which provided an unusually strong connection while controlling the stress and strain in the connected pin and box members of the connection. The thread form featured mating helical threads which were tapered in thread width in opposite directions to provide wedge-like engagement of the opposing flanks to limit rotational make-up of the connection.
The wedge thread, if properly designed, provides high torsional resistance without inducing axial or radial stresses into the tubular connection upon make-up of the joint. Tubular connections with high torsional resistance resist additional make-up in the joint when in service, making it easier to break out the joints if this becomes necessary. By reducing axial or radial stresses in the threaded connection, a sounder connection is provided which is able to withstand a greater level of operating stress and strain. Whereas, traditional threaded connections employed mating threads structures which placed in the pin in hoop compression and the box in hoop tension, the Blose connection controlled the stresses induced in the mating members to pull the two members together, rather than apart.
U.S. Pat. No. 4,600,224, issued Jul. 15, 1986 to Blose was a refinement and further improvement to the basic wedge thread concept. In the invention disclosed in the '224 patent, a connection was shown having a "chevron" load flank. Radial make-up of the threaded connection was controlled by the special thread structuring where the radial movement of a thread into a mating thread groove was restricted by a chevron type interfit between two load bearing thread surfaces of the threaded connection instead of relying upon thread width alone.
Due to the helical wedging action of the threads and the balance of radial components from the forces on the stab and load bearing surfaces of the threads, no radial component of stress was induced in either the pin or box member of the connection. It was also possible to control axial make-up of the threaded connection without abutting a shoulder on the joint through the wedging action of the threads themselves. By controlling the radial interfit between mating threads, lubricant entrapment could be controlled.
Re. Pat. No. 34,467 issued Dec. 7, 1992 to Reeves purported to be an improvement to the basic Blose wedge thread design. As explained by the patentee, when Blose's connection is rotatably made up to engage both the front and back thread load flanks, incompressible thread lubricant or other liquid may be trapped between the engaged load flanks. This trapped thread lubricant can resist the make-up torque and give a false torque indication that results in lower than desired stress and strain being induced in the Blose connection and reducing the design strength and load carrying capacity. The invention described in Re. Pat. No. 34,467 purports to preclude the possibility of false indication of torque by excluding thread lubricant from between the thread load flanks that are brought into engagement at make-up.
In the conventional prior art connection, only a single load flank is typically engaged during make-up and clearance is provided adjacent the back thread flank. By providing thread clearance on the back flank of conventional thread structures, a helical escape flow passage or reservoir is provided for receiving any excess of displaced liquid thread lubricant during make-up. Since Blose's threads were structured helically as a "wedge", it was necessary to provide a radial clearance between the thread crests and roots upon make-up to provide the desired escape path or any entrapped lubricant. Re. Pat. No. 34,467 purports to eliminate the problem of entrapped lubricant by optimizing the geometry of the Blose wedge thread structure.
As stated by the patentee, the root wall on the external thread member contacts the crest walls on the internal thread member to exclude the trapping of liquid therebetween during rotational make-up of the connection. In the earlier Blose connection, the roots and crests of the threads did not engage. Thread lubricant could be trapped in the long helical space between the roots and crests of the thread. In Re. Pat. No. 34,467, the threads were modified so that the roots and crests would engage before the load flanks engaged, thereby causing the thread lubricant between the roots and crests to be squeezed out from between the surfaces before the connection is fully made up.
In Re. Pat. No. 30,647 and Re. Pat. No. 34,467, the preferred threads were "dovetailed-shaped" in cross section, being wider at the crests than at the roots. U.S. Pat. No. 4,600,224 was a departure from the Blose design in that a semi-dovetail or partial dovetail thread was disclosed. However, the thread crest width continued to be greater than the thread root width as in the traditional definition of the term "dovetail."
The present invention has as its object to provide a further modification of the basic wedge thread concept which provides improved performance over the prior art designs.